1. Field of the Invention
The present invention relates to an infrared curable ink composition for a color filter, and more particularly, an infrared curable ink composition for a color filter including a melamine compound and an epoxy compound.
The present application claims priority to Korean Patent Application Nos. 10-2009-0063160 filed on Jul. 10, 2009, 10-2009-0063161 filed on Jul. 10, 2009 and 10-2010-0065873 filed on Jul. 8, 2010, the disclosure of which is incorporated herein by reference in its entirety.
2. Description of the Related Art
Display device is essential for the age of information society and multimedia, and recently, it is made to be small and thin in size and light in weight, thereby being applied to a variety of fields. With the advance of semiconductor technology, the concerns about flat panel display (FPD) applicable to various fields are growing, and there have been developed many different types of flat panel display, including liquid crystal display (LCD), plasma display panel (PDP), organic electroluminiscent display (OELD) or the like.
In the liquid crystal display device, an electric voltage is applied to vary the molecular arrangement of the liquid crystal layer, and therefore light transmittance is controlled to display desired information. To develop color, the liquid crystal display device is provided with color filters of the three colors including red, green and blue. When light transmittance is controlled by altering the liquid crystal arrangement, light passing through each color filter is controlled, such a way that, color is developed.
Typically, color filters are located on a glass substrate, and three types of the color filters should be regularly and elaborately distributed in a specific shape. As the method of forming three types of the color filter, a printing method, an electrodeposition method, a photolithography method, and an ink-jet printing method are reported.
First, the photolithography method is as follows. Pixel regions are defined on a transparent substrate, and black matrices for shielding a light source between the pixel regions are formed on the transparent substrate. Next, a color resist is coated to cover the entire surface of the substrate including the black matrices, and a color filter pattern of a specific color (for example, a red color filter pattern) is formed by exposing the color resist using a mask. Subsequently, a red color filter is formed on the glass substrate through development and curing processes. Sequentially, other color filters are also formed by repeatedly performing the processes. However, this method is problematic in that its efficiency is reduced by large amounts of loss of materials and the process is complicated.
Second, the ink-jet printing method is a method of directly printing color filter materials, and has advantages that the process can be simplified, loss of materials can be prevented, and the process cost can be also reduced without the need of photolithography process. In the ink-jet printing method, a black matrix pattern is formed on the glass substrate, and the formed pixel space is filled with ink including a pigment. The volatile solvent contained in the ink is evaporated through the process of curing ink (baking process), and a color filter is formed by cross-linking process.
The process of manufacturing a color filter is exposed to a lot of chemicals, and thus the formed color filter is required to have chemical resistance during the process. The chemical resistance is determined by the cross-linking density of color filter ink film, and thus the process of curing ink applied on the glass substrate is a very important process during the manufacture of the color filter.
In the conventional process of curing ink, that is, baking process, a post-baking process employing a thermal convection method was used. In this process, the substrate is loaded in a chamber, and heated by thermal convection method, so that cross-linking reaction in the ink occurs and the solvent is volatilized. That is, ink for a color filter is jetted on the glass substrate, and then heating is performed at a predetermined temperature for solvent evaporation, thereby stabilizing the ink on the substrate. However, the post-baking process employing the thermal convection method is problematic in that it must employ a gas becoming a convection mediator within the chamber and requires a significantly long time in order to cure the color filter film. During the baking process, various foreign materials are generated on the substrate, and in particular, during the post-baking process employing the thermal convection method, the possibility of generating foreign materials increases due to a long baking time. Thus, the process has drawbacks that much time and operating costs are required for the production process, resulting in low efficiency.
Therefore, instead of the post-baking process employing the thermal convection method, alternative processes have been developed, and Korean Patent Laid-Open Publication No. 10-2008-0083944 discloses an IR Curing device capable of performing the post-baking process by infrared radiation with high heat transfer.
Unlike the conventional post-baking process using the thermal convection method, the infrared curing process employs infrared rays having a high output and an excellent heating characteristic. In particular, it is characterized by a rapid curing performance, thereby reducing production time and costs, and improving, production yield. Thanks to a short curing time, the generation of foreign materials on the substrate can be also reduced during the curing process.
Unlike the post-baking process using the thermal convection method, the infrared curing process is performed for a short curing time (approximately 1 to 10 min) at high temperature (250° C. or higher). Therefore, the ink for a color filter used in this process is required to have physical and chemical characteristics different from those used in the post-baking process using the thermal convection method.
In general, the ink for a color filter is characterized in that it has a higher pigment concentration and higher viscosity due to high solid content, compared to the photoresist (PR) for a color filter. The chemical resistance of the ink for a color filter is determined by the strength and cross-linking density of ink film. Because of its higher pigment content, a relatively small amount of binder and cross-linking agent are contained, leading to deterioration in the chemical resistance of the ink film. The deterioration in the chemical resistance generates problems that pigments contained in the color filter film are melted out upon post-processing such as alignment film formation during the production of color filter, leading to deterioration in color development or influence on liquid crystal operation. In particular, a red ink is problematically weak in chemical resistance.
Accordingly, the ingredients contained in the ink for a color filter should have excellent chemical resistance, and maintain high cross-linking density even at a small amount, and not deteriorate pigment dispersibility when added to the ink. In addition, those possessing an extremely high viscosity should not be used, because they deteriorate the jetting property. The ink for a color filter used in the infrared curing process has a very short curing time. Thus, if the cross-linking process is not performed rapidly, its chemical resistance is deteriorated. Therefore, the ink for a color filter is required to have a rapid cross-linking characteristic.
The conventional ink compositions for a color filter are described in Japanese Patent Laid-Open Publication Nos. 2009-74010 and 1995-196968. However, these ink compositions for a color filter are not used for the infrared curing process, and not suitable for the 3 minute curing process such as infrared curing process.
Accordingly, there is a need for an ink composition for a color filter having excellent chemical resistance, in which it can be sufficiently cured for a short period of time as in the infrared curing process. Therefore, the present inventors have made studies to develop an ink composition for a color filter, which is suitable for the infrared curing process and has excellent chemical resistance, thereby completing the present invention.